<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-03-15</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000913665</issuerCik>
        <issuerName>TERRESTAR CORP</issuerName>
        <issuerTradingSymbol>TSTRQ</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001279913</rptOwnerCik>
            <rptOwnerName>MARATHON ASSET MANAGEMENT LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>ONE BRYANT PARK, 38TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10036</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2011-03-15</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>850000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.1203</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>21347001</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The shares of Common Stock, par value $0.01 per share (&quot;Shares&quot;), of TerreStar Corporation reported herein are held by one or more funds and accounts (the &quot;Funds and Accounts&quot;) advised by Marathon Asset Management, L.P. (&quot;Marathon&quot;).  Marathon, in its capacity as the investment manager of the Funds and Accounts, has sole power to vote and direct the disposition of all Shares held by the Funds and Accounts.  Thus, for the purposes of Reg. Section 240.13d-3, Marathon may be deemed to beneficially own 21,347,001 Shares.  Marathon's interest in such securities is limited to the extent of its pecuniary interest in the Funds and Accounts, if any.  This report shall not be deemed an admission that Marathon, each Fund and Account or any other person is the beneficial owner of the securities reported herein for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, or for any other purpose.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>by: /s/ Louis Hanover, Authorized Person of Marathon Asset Management GP, L.L.C., general partner of Marathon Asset Management, L.P.</signatureName>
        <signatureDate>2011-03-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
